Ceramic honeycomb structures

ABSTRACT

Ceramic honeycomb structures may include a mullite phase and a tialite phase, and methods for manufacturing ceramic honeycomb structures may include making ceramic honeycomb structures including a mullite phase and a tialite phase. Ceramic honeycomb structures may include andalusite, and methods for manufacturing ceramic honeycomb structures may include using andalusite and making ceramic honeycomb structures including andalusite.

FIELD OF THE INVENTION

The present invention relates to ceramic honeycomb structures comprisinga mullite phase and a tialite phase, to methods for manufacturing thesehoneycomb structures, and to uses of such structures.

The present invention also relates to the use of andalusite for themanufacture of a ceramic honeycomb structure, as well as to the ceramichoneycomb structures containing andalusite, and to the methods formanufacturing these ceramic honeycomb structures.

BACKGROUND OF THE INVENTION

Ceramic honeycomb structures are known in the art for the manufacture offilters for liquid and gaseous media. However, the most relevantapplication today is the use of such ceramic bodies as particle filtersfor the removal of fine particles from the exhaust gas of diesel enginesof vehicles (diesel particulates), since those fine particles have beenshown to have negative influence on human health.

Several ceramic materials have been described for the manufacture ofceramic honeycomb filters suitable for that specific application. Theceramic material has to fulfill several requirements: First, thematerial must show sufficient filtering efficiency, i.e., the exhaustgas passing the filter should be substantially free of dieselparticulates. However, the filter should not produce a substantialpressure drop, i.e., it must show a sufficient ability to let theexhaust gas stream pass through its walls. These properties generallydepend upon the wall parameters (thickness, porosity, pore size, etc.)of the filter.

Furthermore, the material must show sufficient chemical resistanceagainst the compounds present in exhaust gas of diesel engines over abroad temperature range.

Next, the material must be resistant against thermal shock due to thehigh temperature differences that apply during its life cycle. Forexample, the filter is in permanent contact with hot exhaust gas duringthe runtime of the diesel engine; however, there may be a largetemperature difference between the filter and the exhaust gas when theengine is started after a period of inactivity. Additionally, after acertain period of use, the filter is loaded with particulates that mustbe removed to avoid high exhaust gas pressure drop. Normally, thecleaning of the filter is performed by periodically heating the filterto a temperature sufficient to ignite the collected diesel particulatesat high temperatures (>1000° C.). Thus, it is evident that the filtermaterial must show sufficient thermal shock resistance to surviveseveral regeneration cycles comprising rapid heating to temperaturessubstantially higher than the normal operating temperature. A criticalparameter for thermal shock resistance is the thermal conductivity ofthe material.

It is evident from the above that the material must also have a meltingpoint above the temperatures reached within the filter during theregeneration cycle. Additionally, the material should have a low thermalexpansion coefficient to avoid mechanical tensions during the heatingand cooling periods. If the above requirements are not fulfilled,mechanical and/or thermal tension may cause cracks in the ceramicmaterial, resulting in a decrease of filtering efficiency.

Since the filters for vehicles are produced in high numbers, the ceramicmaterial should be inexpensive, and the process for its preparationshould be cost-effective.

A summary on the ceramic materials known for this application is givenin the paper of J. Adler, Int. I Appl. Ceram. Technol. 2005, 2 (6),429-439, the content of which is incorporated herein in its entirety forall purposes.

Several attempts have been made to improve the properties of the ceramicfilter material; however, a ceramic material meeting all of the abovecriteria in an ideal manner has not yet been found. Thus, there is aneed in the art for new ceramic filter materials showing improvedproperties over those of the prior art.

A conventional material known in the art is silicon carbide (SiC).However, this material is expensive and somewhat difficult to handle forthe purpose of diesel particulate filter manufacture.

Honeycombs made from ceramic materials based on mullite and/or tialitehave been used for the construction of diesel particulate filters.Mullite is an aluminum and silicon containing silicate mineral ofvariable composition between the two defined phases [3 Al₂O₃.2 SiO₂](the so-called “stoichiometric” mullite, or “3:2 mullite”) and [2Al₂O₃.1 SiO₂] (the so-called “2:1 mullite”). The material is known tohave a high melting point and fair mechanical properties, but relativelypoor thermal shock resistance.

Another ceramic material that has been explored for its use in the fieldof diesel particulate filters is tialite, an aluminum titanate havingthe formula [Al₂TiO₅]. The material is known to show a high thermalshock resistance, but reduced mechanical strength. The reducedmechanical strength is expressed, e.g., by the Young modulus, which is20 GPa for tialite, but 150 GPa for mullite. Additionally, the materialtends to thermal instability at the operation conditions of a dieselparticulate filter. Moreover, due to its titanium content, it is arelatively expensive material.

Several attempts have been made in the past to combine the positiveproperties of mullite and tialite, e.g., by developing ceramic materialscomprising both phases.

U.S. Pat. No. 5,290,739 describes a crack-free, sintered ceramic articleproduced by (1) pre-reacting or doping mullite with titania, hematite(iron oxide) and/or the precursors of these metal oxides; (2) calciningthe doped mullite; (3) calcining aluminum-titanate; (4) mixing thecalcined, doped mullite and the aluminum-titanate with a binder to forma ceramic batch, which can be optionally shaped, to form a green ceramicarticle; and (5) firing the green ceramic article at a temperature andfor a duration of time sufficient to form the sintered ceramic article.In the example, a green ceramic article having 42.2% mullite and 57.8%aluminum titanate is formed.

WO-A-2004/011124 relates to a diesel exhaust particulate filtercomprising a plugged, wall-flow honeycomb filter body composed of porousceramic material and having a plurality of parallel end-plugged cellchannels traversing the body from a frontal inlet end to an outlet endthereof. The porous ceramic contains, expressed in terms of weightpercent of the total body, of 60-90%, preferably 70-80%, most preferably70% iron-aluminum titanate solid solution having a stoichiometry ofAl_(2(1-x))Fe_(2x)TiO₅, where x is 0-0.1, and 10-40%, preferably 20-30%,most preferably 30% mullite (3Al₂O₃.2SiO₂), and consists essentially,expressed in terms of weight percent on the oxide basis, of 3 to 15%,preferably 6 to 12% SiO₂, 55 to 65%, preferably 57 to 61% Al₂O₃, 22 to40%, preferably 26 to 35% TiO₂, and 0 to 10%, preferably 0.5 to 5%Fe₂O₃. In the example, the ceramic body contains 70 wt % iron-aluminumtitanate solid solution and 30% mullite.

U.S. Pat. No. 4,767,731 describes sintered aluminum titanate-mullitebase ceramics. It is stated that the quantitative relation betweenaluminum titanate starting material and mullite must be that thealuminum titanate starting material is from 40 to 65% and the mulliteraw material is from 35 to 60%. A comparative example is described inwhich the aluminum titanate starting material is 35% and the mullite rawmaterial is 65 wt %. The volume ratio of the mullite to aluminumtitanate in the sintered body is not stated.

It has now been found that a ceramic material providing increasedmechanical strength in combination with high thermal shock resistancecan be manufactured which comprises a high amount of a mullite phase incombination with a minor amount of tialite; i.e., the mullite phase isthe dominant phase. Moreover, it has been found that the thermalinstability of the tialite phase in such combined ceramic materials issubstantially reduced over the prior art compositions.

Moreover, it has been found that the use of andalusite as a raw materialfor the manufacture of ceramic honeycomb structures leads to ceramicmaterials showing improved properties in the above application.

Andalusite is a mineral from the group of the so-called sillimaniteminerals (kyanite, sillimanite, andalusite). All of those minerals showan identical chemical composition, and the corresponding formula isAl₂SiO₅. Thus, the three minerals represent different polymorphic formsof the same composition. When exposed to high temperatures (sinteringconditions of ceramic materials), the sillimanite minerals decomposeunder the formation of two new phases, a glassy silica phase and a“stoichiometric” mullite phase (or “3:2 mullite” phase). While themullite phase is known to be chemically resistant against diesel exhaustgas, the presence of the glassy silica phase is undesirable due to itshigh chemical reactivity with trace components of the exhaust gas.Additionally, although the mullite phase has a high melting point, itsthermal conductivity is relatively poor. The glassy phase has a poorthermal stability due to its low melting point. The present inventionshows how the above disadvantages of ceramic compositions obtained fromandalusite can be overcome. Moreover, it has surprisingly been foundthat the presence of a certain amount of unreacted andalusite in thefinal honeycomb structure provides some advantageous properties for theuse of the structure as a diesel particulate filter.

SUMMARY OF THE INVENTION

The present invention is directed to a ceramic honeycomb structurecomprising a mineral phase of mullite and a mineral phase of tialite,wherein the volume ratio of mullite to tialite is 2:1 or higher, or2.5:1 or higher, or 3:1 or higher, or 4:1 or higher, or 5:1 or higher,or 8:1 or higher, or 10:1 or higher. In one embodiment, the mullitephase is 3:2 mullite. In a further embodiment, the tialite phase isenclosed by the mullite phase. In a further embodiment, the mullite isin the form of crystals which are substantially parallel. In a furtherembodiment, the amount of mullite in the ceramic honeycomb structure isgreater than 50%, or greater than 75%, or greater than 80%, by volume(calculated on the basis of the total volume of the mineral phases ofthe honeycomb). In a further embodiment, the ceramic honeycomb structuremay comprise one or more additional solid mineral phases selected fromthe group consisting of cordierite, andalusite, zirconia, titania, asilica phase, magnesium oxide, magnesia alumina spinel, silicon carbide,and silicon nitride. In another embodiment, the silicon carbide ispresent in the ceramic honeycomb structure in an amount between 4 and30% by mass. In another embodiment of the invention the silicon carbideis present in the ceramic honeycomb structure in an amount between 4 and12%, or between 8 and 12% by mass. The particle size of the siliconcarbide in one embodiment is between 0.5 and 20 microns, or between 1and 15 microns, or between 1 and 10 microns. In another embodiment, themagnesia alumina spinel is present in the ceramic honeycomb structure inan amount between 4 and 30% by mass, or between 4 and 12%, or between 8and 12% by mass. The particle size of the magnesia alumina spinel in oneembodiment is between 0.5 and 20 microns, or between 1 and 15 microns,or between 1 and 10 microns.

In an embodiment, the amount of iron in the honeycomb structure,measured as Fe₂O₃, is less than 5% by weight, and for example may beless than 2% by weight, or for example less than 1% by weight. Thestructure may be essentially free from iron, as may be achieved forexample by using starting materials which are essentially free of iron.Iron content may be measured by XRF.

As noted, the ceramic honeycomb structure may comprise a mineral phaseconsisting of andalusite. In an embodiment, such an andalusite phase ispresent in an amount of up to 10% by volume, or up to 8% by volume, orup to 5% by volume or up to 2% by volume, or up to 0.5% by volume (basedon the volume of the solid phases of the ceramic honeycomb structure).

In a further embodiment, the ceramic honeycomb structure of theinvention is a porous structure, wherein the total pore volume is in therange between 30% and 70%, or between 45% and 65%. In a furtherembodiment, the total pore volume is in the range between 40% and 60%.(The volume percentages are calculated on the basis of the total volumeof mineral phases and pore space.) In a further embodiment, the totalpore volume of the structure is in the range between 30% and 70%, orbetween 40% and 65%, or between 50% and 65% (calculated on the basis ofthe total volume of mineral phases and pore space).

The present invention also provides a method for producing the aboveceramic honeycomb structures, comprising the steps of

-   -   a. providing a dried green honeycomb structure comprising        mullite and/or one or more mullite-forming compounds or        compositions and tialite and/or one or more tialite-forming        compounds or compositions; and    -   b. sintering.

In a further embodiment, the method comprises the steps of

-   -   a. providing a green honeycomb structure comprising mullite        and/or one or more mullite-forming compounds or compositions and        tialite and/or one or more tialite-forming compounds or        compositions;    -   b. drying the green honeycomb structure, and    -   c. sintering.

In a further embodiment, the method comprises the steps of

-   -   a. providing an extrudable mixture comprising mullite and/or one        or more mullite-forming compounds or compositions and tialite        and/or one or more tialite-forming compounds or compositions;    -   b. extruding the mixture to form a green honeycomb structure;    -   c. drying the green honeycomb structure, and    -   d. sintering.

In a further embodiment of the above method, the mullite-formingcomposition is a mullite-forming clay composition. In anotherembodiment, the mullite-forming compound is selected from the groupconsisting of kyanite, andalusite, and sillimanite. In yet anotherembodiment, the mullite-forming compound is andalusite.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises tialite. The tialite can bepresent in an amount of between 2.5% to 10%, or 4% to 7%, by weight (dryweight of the extrudable mixture or the green honeycomb structure). In afurther embodiment, the tialite is present in an amount between 2.5% to15%, or between 5% to 12%, or between 4% to 7% by weight (dry weight ofthe extrudable mixture).

In a further embodiment of the above method, the tialite-formingcomposition is a mixture of titania (TiO₂) and alumina (Al₂O₃). In afurther embodiment, the alumina is present in the form of particleshaving a size in the range between 0.01 to 10 μm, or between 0.01 to 1μm, or between 0.03 to 0.06 μm. In a further embodiment, the alumina isused in the form of colloidal/nanometric solutions. In a furtherembodiment, the titania is present in the form of particles having asize in the range between 0.01 to 10 μm, or between 0.01 to 1 μm, orbetween 0.03 to 0.06 μm. In a further embodiment, the titania is presentin the form of particles having a size in the range between 0.01 to 10μm, or between 0.2 to 1 μm, or between 0.2 to 0.5 μm. In a furtherembodiment, the titania is used in the form of colloidal/nanometricsolutions. Where colloidal titania is used, this may be employedtogether with a non-colloidal form of titania, for example one having ad₅₀ smaller than 1 μm, for example a d₅₀ smaller than 0.5 μm. In afurther embodiment, the size of the titania particles is larger than thesize of the alumina particles. In a further embodiment, the amount ofthe alumina in the raw material is higher than the amount of titania.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises graphite. In another embodiment,the graphite is present in the form of particles having a medianparticle diameter (D50) in the range between 1 and 100 μm, or between 5μm to 50 μm, or between 7 μm and 30 μm, or between 20 μm and 30 μm.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises one or more binding agentsselected from the group consisting of, methyl cellulose,hydroxymethylpropyl cellulose, polyvinyl butyrals, emulsified acrylates,polyvinyl alcohols, polyvinyl pyrrolidones, polyacrylics, starch,silicon binders, polyacrylates, silicates, polyethylene imine,lignosulfonates, and alginates.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises one or more mineral binders.Suitable mineral binder may be selected from the group including, butnot limited to, one or more of bentonite, aluminum phosphate, boehmite,sodium silicates, boron silicates, or mixtures thereof.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises one or more auxiliants (e.g.plasticizers and lubricants) selected from the groups consisting ofpolyethylene glycols (PEGs), glycerol, ethylene glycol, octylphthalates, ammonium stearates, wax emulsions, oleic acid, Manhattanfish oil, stearic acid, wax, palmitic acid, linoleic acid, myristicacid, and lauric acid.

In a further embodiment of the above method, the sintering step is at atemperature between 1250° C. and 1700° C., or between 1350° C. and 1600°C., or between 1400° C. and 1550° C., or between 1400° C. and 1500° C.

Additionally, the present invention is directed to a ceramic honeycombstructure comprising a mineral phase consisting of andalusite. In afurther embodiment, the andalusite phase is present in an amount of 0.5%to less than 50%, or 2% to 50%, or 5% to 30%, or 7% to 20%, orcomprising 0.5% to 15%, or 2% to 14%, or 4% to 10%, or 5% to 8%, byvolume (based on the volume of the solid phases of the ceramic honeycombstructure).

In a further embodiment, the andalusite-containing ceramic honeycombstructure comprises mullite, which may be present in the form of 3:2mullite (stoichiometric mullite). In a further embodiment, the volumeratio between andalusite to mullite in the structure is in the rangebetween 1:10 to 1:99, or between 1:10 to 1:80, or between 1:10 to 1:25(based on the volume of the solid phases of the ceramic honeycombstructure).

In a further embodiment, the andalusite-containing ceramic honeycombstructure comprises tialite. In a further embodiment, the volume ratioof mullite to tialite in the structure is 2:1 or higher, or 2.5:1 orhigher, or 3:1 or higher, or 4:1 or higher, or 5:1 or higher, or 8:1 orhigher, or 10:1 or higher (based on the volume of the solid phases ofthe ceramic honeycomb structure). In a further embodiment, the tialitephase is enclosed by the mullite phase.

In a further embodiment, the andalusite-containing ceramic honeycombstructure is a porous structure, wherein the total pore volume is in therange between 30% and 70%, or between 45% and 65%. In a furtherembodiment, the total pore volume is in the range between 40% and 60%.In a further embodiment, the total pore volume of the structure is inthe range between 30% and 70%, or between 40% and 65%, or between 50%and 65% (calculated on the basis of the total volume of mineral phasesand pore space).

In another embodiment, the andalusite-containing ceramic honeycombstructure may comprise one or more additional solid mineral phasesselected from the group consisting of cordierite, zirconia, titania, asilica phase, magnesium oxide, magnesia alumina spinel (MgAl₂O₄),silicon carbide, and silicon nitride. In another embodiment, the siliconcarbide is present in the andalusite-containing ceramic honeycombstructure in an amount between 4 and 30% by mass. In another embodimentof the invention the silicon carbide is present in theandalusite-containing ceramic honeycomb structure in an amount between 4and 12%, or between 8 and 12% by mass. The particle size of the siliconcarbide in one embodiment is between 0.5 and 20 microns, or between 1and 15 microns, or between 1 and 10 microns. In another embodiment, themagnesia alumina spinel is present in the andalusite-containing ceramichoneycomb structure in an amount between 4 and 30% by mass, or between 4and 12%, or between 8 and 12% by mass. The particle size of the magnesiaalumina spinel in one embodiment is between 0.5 and 20 microns, orbetween 1 and 15 microns, or between 1 and 10 microns.

In one embodiment, the andalusite-containing ceramic honeycomb structurecomprises

-   -   0.5 to 15.0%, or 2 to 12%, or 4 to 10%, or 5 to 8%, or 1 to 6%        of andalusite;    -   60 to 90.0%, or 75.0 to 90.0% of mullite;    -   2.5 to 20.0%, or 12 to 18%, or 2.5 to 10.0%, or 4 to 7%, of        tialite;    -   0 to 2% of rutile and/or anatase; and    -   3.0 to 20.0% of an amorphous silica phase;        wherein the total amount of the above components is 100% by        volume (based on the volume of the solid compounds).

In an embodiment, the amount of iron in the honeycomb structure,measured as Fe₂O₃, is less than 5% by weight, and for example may beless than 2% by weight, or for example less than 1% by weight. Thestructure may be essentially free from iron, as may be achieved forexample by using starting materials which are essentially free of iron.

In a further embodiment, the invention refers to a diesel particulatefilter made using the above ceramic honeycomb structures.

The present invention also provides a method for producing theandalusite-containing ceramic honeycomb structures, comprising the stepsof

-   -   a. providing a dried green honeycomb structure comprising        andalusite; and    -   b. sintering.

In a further embodiment, the method comprises the steps of

-   -   a. providing a green honeycomb structure comprising andalusite;    -   b. drying the green honeycomb structure, and    -   c. sintering.

In a further embodiment, the method comprises the steps of

-   -   providing an extrudable mixture comprising andalusite;    -   extruding the mixture to form a green honeycomb structure;    -   drying the green honeycomb structure; and    -   sintering.

In a further embodiment of the above method, the sintering step is at atemperature between 1250° C. and 1700° C., or between 1350° C. and 1600°C., or between 1400° C. and 1550° C., or between 1400° C. and 1500° C.

In another embodiment of the above method, the method comprises anadditional step of heating the green honeycomb structure to atemperature in the range of 200° C. to 300° C. prior to the sinteringstep.

In yet another embodiment of the above method, the method comprises thestep of heating the green honeycomb structure to a temperature in therange of between 650° C. and 950° C., or between 650° C. and 900° C., orbetween 800° C. and 850° C. prior to the sintering step.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises 50% or more by weight (based onthe dry weight of the raw material) of andalusite.

In a further embodiment of the above method, the andalusite is used inthe form of particles having a size in the range between 0.1 μm and 125μm, or between 0.1 μm and 100 μm, or between 0.1 μm and 75 μm, orbetween 25 μm and 100 μm, or between 25 μm and 75 μm.

In a further embodiment of the above method, the andalusite is used inthe form of particles having a size in the range between 0.1 μm and 55μm, or between 10 μm and 55 μm, or between 15 μm and 55 μm, or between20 μm and 55 μm.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure further comprises alumina and/or titania.With regard to the total amount of alumina, the alumina is present in anamount of 25 to 55%, or 30 to 50%, or 35 to 48% (based on the dry weightof the extrudable mixture or the green honeycomb structure). With regardto the amount of titania, the titania is present in an amount of 0.5 to5%, or 1 to 4%, or 2 to 3.5% (based on the dry weight of the extrudablemixture or the green honeycomb structure). In a further embodiment, thetotal titania content of the extrudable mixture is present in an amountof 0.5 to 10%, or 1 to 8%, or 1 to 6%, or 2 to 4% by weight (dry weightof the extrudable mixture).

In a further embodiment of the above method, the alumina compound ispresent in the form of particles having a size in the range between 0.01to 10 μm, or in the range between 0.01 to 1 μm, or between 0.03 to 0.06μm. In a further embodiment, the alumina is used in the form ofcolloidal/nanometric solutions. In a further embodiment, the titania ispresent in the form or particles having a size in the range between 0.01to 10 μm, or between 0.01 to 1 μm, or between 0.03 to 0.06 μm. In afurther embodiment, the titania is present in the form of particleshaving a size in the range between 0.01 to 10 μm, or between 0.2 to 1μm, or between 0.2 to 0.5 μm. In a further embodiment, the titania isused in the form of colloidal/nanometric solutions. Where colloidaltitania is used, this may be employed together with a non-colloidal formof titania, for example one having a d₅₀ smaller than 1 μm, for examplea d₅₀ smaller than 0.5 μm. In a further embodiment, the size of thetitania particles is larger than the size of the alumina particles. In afurther embodiment, the amount of the alumina in the raw material ishigher than the amount of titania.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises 3:2 mullite.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises tialite. The tialite can bepresent in an amount of between 2.5% to 10%, or between 4% to 7%, byweight (based on the dry weight of the extrudable mixture or the greenhoneycomb structure). In a further embodiment, the tialite is present inan amount between 2.5% to 15%, or between 5% to 12%, or between 4% to 7%by weight (dry weight of the extrudable mixture).

In a further embodiment of the above method, the raw material comprisesa graphite component. The graphite can be present in an amount of 10% to20% by weight (based on the dry weight of the raw material). Thegraphite material can be used in a particulate form, wherein theparticles have a size of less than 200 μm, or less than 150 μm, or lessthan 100 μm. In another embodiment, the graphite particles have a medianparticle diameter (D50) between 0 and 100 μm; or between 5 μm to 50 μm,or between 7 μm and 30 μm, or between 20 μm and 30 μm.

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises one or more binding agentsselected from the group consisting of, methyl cellulose,hydroxymethylpropyl cellulose, polyvinyl butyrals, emulsified acrylates,polyvinyl alcohols, polyvinyl pyrrolidones, polyacrylics, starch,silicon binders, polyacrylates, silicates, polyethylene imine,lignosulfonates, and alginates. In a further embodiment of the abovemethod, the extrudable mixture or the green honeycomb structurecomprises one or more mineral binders. Suitable mineral binder may beselected from the group including, but not limited to, one or more ofbentonite, aluminum phosphate, boehmite, sodium silicates, boronsilicates, or mixtures thereof The binding agents can be present in atotal amount between 1.5% and 15%, or between 2% and 9% (based on thedry weight of the extrudable mixture or the green honeycomb structure).

In a further embodiment of the above method, the extrudable mixture orthe green honeycomb structure comprises one or more one or moreauxiliants (e.g. plasticizers and lubricants) selected from the groupsconsisting of polyethylene glycols (PEGs), glycerol, ethylene glycol,octyl phthalates, ammonium stearates, wax emulsions, oleic acid,Manhattan fish oil, stearic acid, wax, palmitic acid, linoleic acid,myristic acid, and lauric acid. The auxiliants can be present in a totalamount between 1.5% and 15%, or between 2% and 9% (based on the dryweight of the extrudable mixture or the green honeycomb structure; ifliquid auxiliants are used, the weight is included into the dry weightof the extrudable mixture or the green honeycomb structure).

In one embodiment, the extrudable mixture has the following composition:

-   -   50 to 80%, or 50 to 75%, or 50 to 60% (wt-%) of andalusite;    -   0.5 to 10%, or 1 to 4%, or 2 to 3.5% (wt-%) of titania (TiO₂);    -   1 to 10 wt-% of alumina (Al₂O₃);    -   10 to 20 wt-% of graphite;    -   2 to 9 wt-% of binder; and    -   a total of 2 to 9 wt-% of one or more auxiliants;    -   wherein the total amount of the above components is 100 wt-%        (dry weight of the extrudable mixture).

In an embodiment, the composition may include an amount of iron which isless than 5wt %, measured as Fe₂O₃, or less than 2% by weight, or lessthan 1% by weight. The structure may be essentially free from iron, asmay be achieved for example by using starting materials which areessentially free of iron.

The present invention also refers to the use of andalusite for themanufacture of a ceramic honeycomb structure, wherein the andalusite ispresent in the extruded green honeycomb structure in an amount of 50% ormore (weight-% based on the dry weight of the raw material).

Additionally, the present invention is directed to a raw material forthe manufacture of a ceramic honeycomb structure, comprising at least50%, or between 50% and 55%, by weight of andalusite (based on the dryweight of the raw material). In a further embodiment, the andalusite ispresent in the raw material in the form of particles present in a sizein the range between 0.1 μM and 55 μm, or between 10 μm and 55 μm, orbetween 15 μm and 55 μm, or between 20 μm and 55 μm. In a furtherembodiment, the andalusite is in the form of particles having a size inthe range between 0.1 μm and 125 μm, or between 0.1 μm and 100 μm, orbetween 0.1 μm and 75 μm, or between 25 μm and 100 μm, or between 25 μmand 75 μm. In a further embodiment, the raw material is suitable forextrusion. In a further embodiment, the andalusite is used in the rawmaterial in combination with alumina. In a further embodiment, theandalusite is used in the raw material in combination with titania.

In another embodiment, the invention refers to the use of andalusite forthe manufacture of a ceramic honeycomb structure, wherein the andalusiteis present in an amount between 0.5% to 50%, or 0.5% to 15%, or 2% to14%, or 4% to 10%, or 5% to 8%, by volume (based on the volume of thesolid compounds) of the ceramic honeycomb structure. In a furtherembodiment, the andalusite is present in the green honeycomb structurein the form of particles are present in a size in the range between 0.1μm and 55 μm, or between 10 μm and 55 μM, or between 15 μm and 55 μm, orbetween 20 μm and 55 μm. In a further embodiment, the andalusite is inthe form of particles having a size in the range between 0.1 μm and 125μm, or between 0.1 μm and 100 μm, or between 0.1 μm and 75 μM, orbetween 25 μm and 100 μm, or between 25 μm and 75 μm.

The present invention also refers to the use of alumina (Al₂O₃) for themanufacture of a ceramic honeycomb structure, characterized in that theparticle size of the alumina is in the range between 0.1 to 10 μm, orbetween 0.1 to 1 μm, or between 0.03 μm to 0.06 μm.

The present invention also refers to the use of titania (TiO₂) for themanufacture of a ceramic honeycomb structure, characterized in that theparticle size of the titania is in the range between 0.01 to 10 μm, orbetween 0.2 to 1 μm, or 0.2 μm to 0.5 μm. In a further embodiment, theparticle size of the titania is in the range between 0.1 to 10 μm, orbetween 0.1 to 1 μm, or between 0.03 μm to 0.06 μm. In an embodiment,the titania may be a colloidal titania. The colloidal titania may beused in combination with a non-colloidal titania as noted above.

In another embodiment, the invention refers to the use of alumina ortitania for the manufacture of a ceramic honeycomb structure, whereinthe alumina is used in the form of a nanometric or colloidal solution.In a further embodiment, alumina and titania are used in combination,wherein the particle size of the titania particles is larger than theparticle size of the alumina. In a further embodiment, the alumina ortitania are used in combination with andalusite. In a furtherembodiment, alumina and titania are used in the form of nanometric orcolloidal solutions.

The present invention also refers to a mixture of one or more mineralsfor the manufacture of a ceramic honeycomb structure, comprising 50% ormore by weight (dry weight of the raw material) of andalusite. In afurther embodiment, the mixture further comprises alumina and/ortitania.

In all of the above embodiments comprising the use of alumina (Al₂O₃),the alumina may be partially of fully replaced by alumina precursorcompounds. By the term “alumina precursor compounds”, such compounds areunderstood which may comprise one or more additional components toaluminum (Al) and oxygen (O), which additional components are removedduring subjecting the alumina precursor compound to sinteringconditions, and wherein the additional components are volatile undersintering conditions. Thus, although the aluminum precursor compound mayhave a total formula different from Al₂O₃, only a component with aformula Al₂O₃ (or its reaction product with further solid phases) isleft behind after sintering. Thus, the amount of alumina precursorcompound present in an extrudable mixture or green honeycomb structureaccording to the invention can be easily recalculated to represent aspecific equivalent of alumina (Al₂O₃). Examples for alumina precursorcompounds include, but are not limited to aluminum salts such asaluminum phosphates, and aluminum sulphates, or aluminum hydroxides suchas boehmite (AlO(OH) and gibbsite (Al(OH)₃). The additional hydrogen andoxygen components present in those compounds are set free duringsintering in form of water. Usually, alumina precursor compounds aremore reactive in solid phase reactions occurring under sinteringconditions, than alumina (Al₂O₃) itself Moreover, several of the aluminaprecursor compounds are available in preparations showing very smallparticle sizes, which also leads to an increased reactivity of theparticles under sintering conditions. In one embodiment, the aluminaprecursor compound is boehmite.

In the ceramic honeycomb structures described in the above embodiments,the optimal pore diameter is in the range between 5 to 30 μM, or 10 to25 μm. Depending on the intended use of the ceramic honeycombs, inparticular with regard to the question whether the ceramic honeycombstructure is further impregnated, e.g., with a catalyst, the abovevalues may be varied. For non-impregnated ceramic honeycomb structures,the pore diameter is usually in the range between 7 and 15 μm, while forimpregnated structures, the range is usually between 20 and 25 μm priorto impregnating. The catalyst material deposited in the pore space willresult in a reduction of the original pore diameter.

The honeycomb structure of the invention can typically include aplurality of cells side by side in a longitudinal direction that areseparated by porous partitions and plugged in an alternating (e.g.,checkerboard) fashion. In one embodiment, the cells of the honeycombstructure are arranged in a repeating pattern. The cells can be square,round, rectangular, octagonal, polygonal or any other shape orcombination of shapes that are suitable for arrangement in a repeatingpattern. Optionally, the opening area at one end face of the honeycombstructural body can be different from an opening area at the other endface thereof. For example, the honeycomb structural body can have agroup of large volume through-holes plugged so as to make a relativelylarge sum of opening areas on its gas inlet side and a group of smallvolume through-holes plugged so as to make a relatively small sum ofopening areas on its gas outlet side.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an electron scan of a section through a ceramic honeycombof Example 1.

FIG. 2 shows an electron scan of a section through a ceramic honeycombof Example 1.

DETAILED DESCRIPTION OF THE INVENTION Definitions And Methods

The “dry weight” of the extrudable mixture or of the green honeycombstructure refers to the total weight of any compounds discussed hereinto be suitable to be used in the extrudable mixture, i.e., the totalweight of the mineral phases and of the binders/auxiliants. The “dryweight” is thus understood to include such auxiliants that are liquidunder ambient conditions, but it does not include water in aqueoussolutions of minerals, binders or auxiliants if such are used to preparethe mixture.

The term “total alumina” of a mixture refers to the amount of Al₂O₃present in mineral phases consisting solely of Al₂O₃, as well as toAl₂O₃ present in other mineral phases, such as andalusite.

The term “total titania” of a mixture refers to the amount of TiO₂present in mineral phases consisting solely of TiO₂, as well as to TiO₂present in other mineral phases, such as andalusite.

The “total volume of the mineral phases” of a ceramic honeycombstructure refers to the volume of the honeycomb without the pore volume,i.e., only solid phases are considered. The “total volume of the mineralphases and pore space” refers to the apparent volume of the ceramichoneycomb body, i.e. including solid phases and pore volume.

The “particle sizes” and “median particle diameters” of particulategraphite, as used herein is determined by measurements by laserdiffraction spectroscopy (Malvern).

The particle size of the andalusite starting material referred to hereinrepresents a range of particle diameters (esd) as measured using aSedigraph 1500 using the standard protocol. In each case, the lowerlimit of a range is the d₁₀ value and the upper limit of the range isthe d₉₀ value.

The particle sizes of the alumina and titania starting materialsreferred to herein represent ranges of particle diameters (esd) asmeasured using static light scattering, for example using a HoribaLA-910 device. In each case, the lower limit of a range is the d₁₀ valueand the upper limit of the range is the d₉₀ value. In the case ofcolloidal titania, such as the S5-300A material used herein, which isquoted by the manufacturer to have a particle size of 30-60 nm, theparticle size is measured using transmission electron microscopy.

The amounts of mullite, tialite and other mineral phases present in theceramic honeycomb structure may be measured using qualitative X-raydiffraction (Cu Kα radiation, Rietveld analysis with a 30% ZnOstandard), or any other measurement method which gives an equivalentresult. As will be understood by the skilled person, in the X-raydiffraction method, the sample is milled and passed completely through a45 μm mesh. After milling and sieving, the powder is homogenized, andthen filled into the sample holder of the X-ray diffractometer. Thepowder is pressed into the holder and any overlapping powder is removedto ensure an even surface. After placing the sample holder containingthe sample into the X-ray diffractometer, the measurement is started.Typical measurement conditions are a step width of 0.01°, a measurementtime of 2 seconds per step and a measurement range from 5 to 80° 2≡. Theresulting diffraction pattern is used for the quantification of thedifferent phases, which the sample material consists of, by usingappropriate software capable of Rietveld refinement. A suitablediffractometer is a SIEMENS D500/501, and suitable Rietveld-Software isBRUKER AXS DIFFRAC^(plus) TOPAS.

The d₅₀ or D50 as used herein refers to the mass median particle size,and is the particle diameter that divides the frequency distribution inhalf, so that 50% of the mass is in particles having a larger diameterand 50% of the mass is in particles having a smaller diameter. The d₁₀and d₉₀ are to be understand in similar fashion.

The measurement of the particle sizes of components which are present inthe sintered honeycomb structure in a particulate form, such as siliconcarbide and magnesium alumina spinel may be accomplished by imageanalysis.

Preparation of Raw Materials

The solid mineral compounds suitable for use as raw materials in thepresent invention (andalusite, alumina, titania, graphite, mullite,tialite, etc.) can be used in the form of powders, suspensions,dispersions, and the like, for the use according to the presentinvention. Corresponding formulations are commercially available andknown to the person skilled in the art. For example, powdered andalusitehaving a particle size range suitable for the present invention iscommercially available under the trade name Kerphalite® (Damrec),powdered graphite having a particle size range suitable for the presentinvention is available under the trade name Timrex® (Timcal), powderedalumina and alumina dispersions are available from Degussa, and powderedtitania and titania dispersions are available from Millennium Chemicals.If necessary, selected cuts of commercially available materials may bemade by techniques known in the art, for example a classificationtechnique such as sieving.

Alternatively, mullite and tialite may be prepared by reacting suitablemineral precursor compounds at high temperatures by methods known to theperson skilled in the art. A summary is given in the textbook of W.Kollenberg (ed.), Technische Keramik, Vulkan-Verlag, Essen, Germany,2004, the content of which is incorporated herein by reference in itsentirety.

The most important function of graphite in the raw materials of thepresent invention is the activity as a pore forming agent during theheating of the green honeycomb structure: At a temperature level aboveabout 650° C. graphite starts to combust, resulting in free pore spacein the final ceramic honeycomb structure where the graphite particleshad been located in the green honeycomb structure. Thus, the amount andthe size distribution of the graphite particles used in the presentinvention is an important parameter to control the total porosity of thefinal ceramic honeycomb structure. Another function of the graphite isto assist in extrusion, functioning as a lubricant.

The binding agents and auxiliants used for the present invention arealso all commercially available from various sources known to the personskilled in the art.

The function of the binding agent is to provide a sufficient mechanicalstability of the green honeycomb structure in the process steps beforethe heating or sintering step. The additional auxiliants provide the rawmaterial with advantageous properties for the extrusion step(plasticizers, glidants, lubricants, and the like).

The preparation of an extrudable mixture from the mineral compounds(optionally in combination with binders and/or auxiliants) is performedaccording to methods and techniques known in the art. For example, theraw materials can be mixed in a conventional kneading machine withaddition of a sufficient amount of a suitable liquid phase as needed(normally water) to obtain a paste suitable for extrusion. Additionally,conventional extruding equipment (such as, e.g., a screw extruder) anddies for the extrusion of honeycomb structures known in the art can beused. A summary on the technology is given in the textbook of W.Kollenberg (ed.), Technische Keramik, Vulkan-Verlag, Essen, Germany,2004, which is incorporated herein by reference.

The diameter of the green honeycomb structures can be determined byselecting extruder dies of desired size and shape. After extrusion, theextruded mass is cut into pieces of suitable length to obtain greenhoneycomb structures of desired format. Suitable cutting means for thisstep (such as wire cutters) are known to the person skilled in the art.

The extruded green honeycomb structure can be dried according to methodsknown in the art (e.g., microwave drying, hot-air drying) prior tosintering. Alternatively, the drying step can be performed by exposingthe green honeycomb structure to an atmosphere with controlled humidityat predefined temperatures in the range between 20° C. and 90° C. overan extended period of time in a climate chamber, where the humidity ofthe surrounding air is reduced in a step-by-step manner, while thetemperature is correspondingly increased. For example, one dryingprogram for the green honeycomb structures of the present invention isas follows:

-   -   maintaining a relative air humidity of 70% at room temperature        for two days;    -   maintaining a relative air humidity of 60% at 50° C. for three        hours;    -   maintaining a relative air humidity of 50% at 75° C. for three        hours; and    -   maintaining a relative air humidity of 50% at 85° C. for twelve        hours.

Heating

The dried green honeycomb structure is then heated in a conventionaloven or kiln for preparation of ceramic materials. Generally, any ovenor kiln that is suitable to subject the heated objects to a predefinedtemperature is suitable for the process of the invention.

When the green honeycomb structure comprises organic binder compoundand/or organic auxiliants, usually the structure is heated to atemperature in the range between 200° C. and 300° C. prior to heatingthe structure to the final sintering temperature, and that temperatureis maintained for a period of time that is sufficient to remove theorganic binder and auxiliant compounds by means of combustion (forexample, between one and three hours).

Additionally, when the green honeycomb structure comprises porousgraphite as a pore forming agent, the structure is heated to atemperature in the range of 650° C. to 900° C. prior to heating thestructure to the final sintering temperature, and that temperature ismaintained for a period that is sufficient to remove the graphiteparticles by means of combustion (for example, between two and fourhours).

For example, one heating program for the manufacture of ceramichoneycomb structures of the present invention is as follows:

-   -   heating from ambient temperature to 250° C. with a heating rate        of 0.5° C./min;    -   maintaining the temperature of 250° C. for two hours;    -   heating to 850° C. with a heating rate of 1.0° C./min;    -   maintaining the temperature of 850° C. for eight hours;    -   heating to the final sintering temperature with a heating rate        of 2.0° C./min; and    -   maintaining the final sintering temperature for about 1 hour to        about three hours.

Sintering

The honeycomb structure may be sintered at a temperature in the rangefrom between 1250° C. and 1700° C., or between 1350° C. and 1600° C., orbetween 1400° C. and 1550° C., or between 1400° C. and 1500° C.

For the embodiments of the invention comprising mullite-formingcomponents/compositions and/or tialite-forming compositions, the abovecomponents/compositions undergo chemical reactions resulting in theformation of mullite and/or tialite. These reactions, as well as therequired reaction conditions, are known to the person skilled in theart. A summary is given in the textbook of W. Kollenberg (ed.),Technische Keramik, Vulkan-Verlag, Essen, Germany, 2004, the contents ofwhich are hereby incorporated by reference in their entirety.

For the embodiments of the invention comprising a tialite-formingcompositions of alumina and titania, it has been surprisingly found thatthe mechanical stability of the ceramic honeycomb structures isincreased, when the titania component is present in the form ofparticles which are larger in size than the alumina particles.

For the embodiments of the invention where the extrudable mixture orgreen honeycomb structure comprises andalusite, during the sinteringstep the andalusite particles in the raw material decompose to formstoichiometric mullite and glassy silica. In the early stages ofmullitization at 1300° C., coarse andalusite crystals dominate, withsmaller amounts of mullite and a glassy phase also existing. Themullitization begins at the crystal edges and cracks. It then progressestoward the pure andalusite zones. At higher temperatures, themullitization progresses until either the mullitization is stoppedbefore full conversion of the andalusite is achieved, or mullitizationis complete. The mullite crystals formed during the mullitization ofandalusite are aligned substantially in parallel with the samecrystallographic orientation—a feature which is believed to enhance thestrength of the resultant material. As used in this application, a“substantially parallel” crystal structure refers to a structure that isapproximately parallel, but not necessarily parallel, and includes, forexample, the crystalline structure that results when andalusitemullitizes. Those skilled in the art will appreciate that determiningthe crystallographic orientation may be conducted by standard opticalexamination or examination under a scanning electron microscope.

Generally, it is possible to maintain the sintering process until allandalusite particles are decomposed. However, for those embodiments ofthe invention where the final ceramic honeycomb structure comprises someandalusite, it is necessary that the sintering process is stopped beforeall the andalusite particles are totally decomposed. The required totaltime for the sintering step depends on the size and shape of the greenhoneycomb structure, the amount of andalusite present in the greenhoneycomb structure and the desired amount of andalusite in the finalceramic honeycomb structure.

The selected sintering temperature, and the oven/kiln used for theprocess, can be easily determined by the skilled person.

Surprisingly, it has been found that the use of andalusite particles ofselected size (such as of a particle size of between 0.1 μm and 55 μm,or between 10 μm and 55 μm, or between 15 μm and 55 μm or between 20 μmand 55 μM; or such as a particle size of between 0.1 μm and 125 μm, orbetween 0.1 μm and 100 μm, or between 0.1 μm and 75 μm, or between 25 μmand 100 μm, or between 25 μm and 75 μm.) has a positive influence on theaverage pore diameter of the final product: in the presence of fineandalusite particles, the average pore diameter is substantiallydecreased. An additional effect of the use of andalusite with a reducedfines content in the above particle size ranges is the reduced shrinkageof the honeycomb structure during the sintering step. Finally, theamount of amorphous glassy phase in the final ceramic honeycombstructure is reduced by using andalusite within the above particle sizeranges.

While the mullite phase is stable under the sintering conditions, theglassy silica phase from the decomposed andalusite is able to react withthe alumina phase of the raw material to form additional stoichiometricmullite. However, in the absence of a sufficient amount of alumina, someof the amorphous glassy phase may remain in the sintered honeycombstructure.

Ceramic Honeycomb Structures

The mullite phase is the major phase forming the backbone of the ceramichoneycomb structure.

In cases where the extrudable mixture or the green honeycomb structurecontains an excess of titania, the final ceramic honeycomb may contain aminor amount of titania, normally in the form of rutile and/or anatase.However, it is desirable to keep the amount of rutile/anatase in a lowrange (i.e., 2% by volume or less).

The ceramic honeycomb structures comprising andalusite according to thepresent invention and prepared according to the processes of the presentinvention usually comprise the following mineral phases:

-   -   andalusite [Al₂SiO₅];    -   stoichiometric mullite [3 Al₂O₃.2 SiO₂];    -   tialite [Al₂TiO₅];    -   an amorphous glassy phase of variable composition, and    -   optionally a low amount of rutile and/or anatase.

The presence of the tialite phase improves the thermal shock resistanceof the ceramic structure. While tialite is normally unstable attemperatures below 1350° C., it has been found that in the structure ofthe ceramic honeycomb materials of the present invention, the tialitephase is enclosed by the stoichiometric mullite phase, resulting in astabilization of the tialite phase at temperature conditions below 1350°C. However, a large amount of tialite in the final ceramic honeycombstructure should be avoided, since the tialite compound has not the samemechanical resistance as mullite. Thus, a high amount of tialite maydecrease the resistance against mechanical tensions of the ceramichoneycomb structure. A reasonable balance is given if the ratio betweenthe mullite phase and the tialite phase is 2:1 or higher, or 2.5:1 orhigher, or 3:1 or higher, or 4:1 or higher, or 5:1 or higher, or 8:1 orhigher, or 10:1 or higher.

The andalusite phase is present in the structure of the ceramichoneycomb materials of the present invention as a residual phase withinthe mullite phase. However, it has surprisingly been found that thepresence of this residual andalusite provides the material with someself-repairing properties for the use as diesel particulate filter: Ifthe ceramic honeycomb structure is damaged by cracks during the lifetimeof the filter, the surface of the crack exposes fresh andalusite. Duringthe regeneration cycle of the filter, the andalusite decomposes intomullite and amorphous silica, which can be melted due to the hightemperatures in the regeneration phase. The liquid amorphous compound isable to move into the crack and become solid during the normal operationof the filter (due to the reduced temperatures). The andalusite phasemay thus be considered to act as an “internal sealing system” for theceramic structure, resulting in an enlarged filtering capacity and/orextended lifetime of the filter. Moreover, since the mullitization ofandalusite is an endothermic process, the progressive mullitization ofthe remaining andalusite during the life cycle of the ceramic honeycombresults in a more efficient behavior during peaks of thermal stress. Theamount of andalusite in the final ceramic honeycomb structures isexpressed by the ratio between the andalusite phase and the mullitephase, which is in the range of 1:10 to 1:99, or in the range of 1:10 to1:80, or 1:10 to 1:25.

In other embodiments, the andalusite is fully converted to mullite, thusleaving no residual andalusite. In experiments, it has been found thatthe complete mullitization of the andalusite gives rise to a materialwhich has a higher breaking strength.

The presence of a silicon carbide component in the ceramic honeycombstructure results in an increase of the thermal conductivity. A similareffect can be reached by the presence of magnesia alumina spinel. Bothcompounds thus increase the thermal shock resistance of the ceramichoneycomb structure, resulting in e.g. a reduction of cracking due tothermal stress.

Total porosity, median pore size and wall thickness of the ceramichoneycomb structure are parameters that may be optimized dependentlyfrom each other to arrive at a honeycomb structure with maximal thermaland mechanical stability together with a minimal back pressure duringits use as a diesel particulate filter.

Further Processing

For the use as diesel particulate filters, the ceramic honeycombstructures of the present invention, or the green ceramic honeycombstructures of the present invention can be further processed byplugging, i.e., close certain open structures of the honeycomb atpredefined positions with additional ceramic mass. Plugging processesthus include the preparation of a suitable plugging mass, applying theplugging mass to the desired positions of the ceramic or green honeycombstructure, and subjecting the plugged honeycomb structure to anadditional sintering step, or sintering the plugged green honeycombstructure in one step, wherein the plugging mass is transformed into aceramic plugging mass having suitable properties for the use in dieselparticulate filters. It is not required that the ceramic plugging massis of the same composition as the ceramic mass of the honeycomb body.Generally, methods and materials for plugging known to the personskilled in the art may be applied for the plugging of the honeycombs ofthe present invention.

The plugged ceramic honeycomb structure may then be fixed in a boxsuitable for mounting the structure into the exhaust gas line of adiesel engine.

EXAMPLES Example 1 Preparation of Extrudable Mixtures And CeramicHoneycomb Structures of the Invention Step 1: Preparation of ExtrudableMixtures, Extruding Process

The raw materials listed in Table 1 were mixed in a conventional mixer(Eirich mixer) to obtain an extrudable paste that is extruded through aconventional extruder (Dorst V15 or V20 extruder) provided with asuitable die to obtain green honeycomb bodies.

TABLE 1 Composition for extrudable mixtures FIDI 24 FIDI 25 With Withselected selected FIDI 20 FIDI 23 Andalusite Andalusite Raw materials %% >10 μm-55 μm >20 μm-55 μm Andalusite 53.8%  53.9% 53.9% 53.9% Graphite15.0%  15.0% 15.0% 15.0% Binder 3.0%  3.0%  3.0%  3.0% H₂O 4.0%  0.0%  0%   0% Titania dispersion 4.0% 12.4% 12.4% 12.4% Alumina dispersion15.0%  10.5% 10.5% 10.5% Auxiliant (plasticizer and lubricant) 5.2% 5.2%  5.2%  5.2% Total: 100.00%   100.00%  100.00%  100.00% 

Andalusite is used in the form of the commercially available productKerphalite® KF (Damrec). Graphite is used in the form of thecommercially available product Timrex® KS75 (Timcal). Titania is used inthe form of the commercially available product S5-300A (MilleniumChemicals, dispersion containing 20% by weight TiO₂). Alumina is used inthe form of the commercially available product Aerodisp® W 630 (Degussa,dispersion containing 30% by weight Al₂O₃). The percentages in the abovetable refer to the total mass amount of each compound in the extrudablemixture.

The mass leaving the extruder was cut with a wire cutter to obtain greenhoneycomb structures having a length of 180 millimeters.

Step 2: Drying

The resulting honeycomb structures were subjected to a drying process ina climate chamber according to the following program:

-   -   maintaining a relative air humidity of 70% at room temperature        for two days;    -   maintaining a relative air humidity of 60% at 50° C. for three        hours;    -   maintaining a relative air humidity of 50% at 75° C. for three        hours; and    -   maintaining a relative air humidity of 50% at 85° C. for twelve        hours.

After the drying step, the dried green honeycomb structures had a lengthof 154 millimeters.

Step 3: Heating And Sintering

The dried green honeycomb structures were subjected to the followingheating program:

-   -   heating from ambient temperature to 250° c with a heating rate        of 0.5° C./min;    -   maintaining the temperature of 250° C. for two hours;    -   heating to 850° C. with a heating rate of 1.0° C./min;    -   maintaining the temperature of 850° C. for eight hours;    -   heating to the final sintering temperature with a heating rate        of 2.0° C./min; and    -   maintaining the final sintering temperature for two hours.

After the sintering step, the ceramic honeycomb had a length of 152millimeters or 6 inches.

The characteristics of the final honeycomb structures are summarized inTable 2:

TABLE 2 Properties of the ceramic honeycomb structures obtained from theextrudable mixtures of Table 1. Formulation FIDI20 FIDI20 FIDI23 FIDI24FIDI24 FIDI25 Final sintering 1450 1550 1450 1450 1500 1450 T (° C.)Total Porous 48 44 46 49.5 48 50 volume (%) Average Pore 3.2 4 4.2 10 1015.4 diameter (μm) Shrinkage after 8 12.3 6 6 6 sintering (%) *RUL -1361 T_(0,5%) (° C.) Crystal phase Mullite 3:2 82 84 78.6 83.3 87.5 84.5(%) Andalusite 3 0 1.2 6.2 0.7 5.2 (%) Rutile — — 0.4 0.1 0.7 0.4 (%)Tialite Al₂TiO₅ — — 2.6 6.4 2.7 5.7 (%) *Refractoriness Under Load 0.2N/mm²

FIG. 1 shows an electron scan of a section from a ceramic honeycombaccording to Example 1, FIDI 24, sintered at 1450° C. The black partsrepresent the pore space of the ceramic honeycomb, while the mullite andandalusite phases are shown in a dark gray tone. The borderline betweenthe two phases can be recognized with the slight difference ofhomogeneity of the material. The andalusite phase can be seen as auniform, slightly darker area in the center of bigger grains. Whereasthe surrounding areas mullite, which can be recognized due to the veinsof light gray tone of glassy silica, which are present inside. Betweenthe grains the light gray phase which intercepts the grains is amorphousglassy phase. The tialite phase is seen in the form of the tinybrightest spot enclosed by the mullite phase.

The composition is further specified by the analytical data provided inTable 3. The datapoints refer to the positions of the section shown inFIG. 2.

TABLE 3 X-ray spectroscopic analysis of Example Composition ChemicalSpectrum Al₂O₃ SiO₂ K₂O CaO TiO₂ Sp 1 - Andalusite 61.91 38.09 Sp 2 -Andalusite 61.75 38.25 Sp 3 - Mullite 64.15 33.84 2.01 Sp 4 - Mullite62.39 35.65 1.96 Sp 5 - Amorphous phase 6.61 84.46 0.92 0.73 7.28 Sp 6 -Amorphous phase 6.78 84.22 0.75 1.09 7.17 Sp 7 - Tialite 56.93 43.07

In order to obtain the X-ray spectroscopic data, the following generalprocedure was followed:

-   1. The sample is embedded in an acrylic resin for metallographic    operations (Struers: SpeciFast hot mounting resin) pressed at 15 kN,    180° C. for 20 mm specimen (Struers prompt press-20)-   2. Polish the specimen with polishing slurries up to 1 my fineness    (polisher: Struers Tegro Pol 35)-   3. Sputter with carbon (Sputtering chamber: BIO-RAD CA 508)-   4. Transfer inward to a Scanning Electron Microscope JSM 6400 (Jeol)-   5. Observe the sample and fixing the areas which have to be analysed    by EDS-   6. Run the EDS analysis of emitted characteristic X-rays with the    analyser OXFORT INCA ENERGY-   7. The analyses, mappings and pictures are done with INCA “The    micronanlyse Suite Issue 16”.

Example 2 Preparation of Extrudable Mixtures According To the Invention

The raw materials listed in Table 4 were mixed in a conventional mixer(Eirich mixer) to obtain an extrudable paste that is extruded through aconventional extruder (Dorst V15 or V20 extruder) provided with asuitable die to obtain green honeycomb bodies.

TABLE 4 Composition for raw materials FIDI 30 FIDI 30 With selected Withselected Andalusite Andalusite Raw materials >10 μm-55 μm >20 μm-55 μmAndalusite 53.8% 53.8% Graphite 13.0% 13.0% Binder  5.0%  5.0% H₂O   0%  0% Titania dispersion 12.4% 12.4% Alumina dispersion 10.5% 10.5%Auxiliant (plasticizer and lubricant)  5.2%  5.2% Total: 100.00% 100.00% 

Andalusite is used in the form of the commercially available productKerphalite® KF (Damrec). Graphite is used in the form of thecommercially available product Timrex® KS75 (Timcal). Titania is used inthe form of the commercially available product S5-300A (MilleniumChemicals, dispersion containing 20% by weight TiO₂). Alumina is used inthe form of the commercially available product Aerodisp® W 630 (Degussa,dispersion containing 30% by weight Al₂O₃). The percentages in the abovetable refer to the total mass amount of each compound in the extrudablemixture.

The resulting mixtures were extruded and further treated as describedabove in Example 1.

Example 3 Preparation of Extrudable Mixtures According To the InventionComprising Silicon Carbide

The raw materials listed in Table 5 were mixed in a conventional mixer(Eirich mixer) to obtain an extrudable paste that is extruded through aconventional extruder (Dorst V15 or V20 extruder) provided with asuitable die to obtain green honeycomb bodies.

TABLE 5 Composition for raw materials: FIDI 21 Raw materials % weightAndalusite 55 μm 50.0%  Graphite 15.0%  Silicon carbide F800 5.0% Binder3.0% H₂O 6.8% Titania dispersion 7.0% Alumina dispersion 8.0% Auxiliant(plasticizer and lubricant) 5.2% Total: 100.00%  

Andalusite is used in the form of the commercially available productKerphalite® KF (Damrec). Graphite is used in the form of thecommercially available product Timrex® KS75 (Timcal). Titania is used inthe form of the commercially available product S5-300A (MilleniumChemicals, dispersion containing 20% by weight TiO₂). Alumina is used inthe form of the commercially available product Aerodisp® W 630 (Degussa,dispersion containing 30% by weight Al₂O₃). The percentages in the abovetable refer to the total mass amount of each compound in the extrudablemixture.

The resulting mixtures were extruded and further treated as describedabove in Example 1.

Example 4 Influence of Particle Size of Alumina Raw Materials

The raw materials listed in Table 6 were mixed in a conventional mixer(Eirich mixer) to obtain an extrudable paste that is extruded through aconventional extruder (Dorst V15 or V20 extruder) provided with asuitable die to obtain green honeycomb bodies.

TABLE 6 Composition for raw materials: FIDI 12 FIDI 14b Raw materials %weight % weight Andalusite 50.0% (45-75 μm) 56.7% (55 μm) Graphite 18.0%18.0% Alumina 9.9% 9.9% Binder 1.5% 1.8% H₂O 18.0% 10.3% Auxiliant(plasticizer and 4% 4% lubricant) Total: 100.0% 100.0%

Andalusite is used in the form of the commercially available productKerphalite® KF (Damrec). Graphite is used in the form of thecommercially available product Timrex® KS 5-44 (Timcal). For FIDI 12,alumina is used in the form of the commercially available productNabalox® N013 (Nabaltec, mean particle size D50 of 0.13 μm). For FIDI14, alumina is used in the form of the commercially available productLocron L® (Clariant, nanocolloidal alumina prepared from atomizedaluminum chlorohydrate).

The resulting mixtures were extruded and further treated as describedabove in Example 1. The crushing resistance of FIDI 12 is 9.6 N; thecrushing resistance of FIDI 14b is 92.6 N.

Example 5 Use of Coarsely Grained Andalusite

The raw materials listed in Table 7 were mixed in a conventional mixer(Eirich mixer) to obtain an extrudable paste that is extruded through aconventional extruder (Dorst V15 or V20 extruder) provided with asuitable die to obtain green honeycomb bodies.

TABLE 7 Composition for raw materials: Raw materials FIDI04 FIDI05Andalusite (200 mesh) 64.12% 61.28% Graphite 14.61% 14.68% Binder 1.46%2.45% H₂O 17.18% 18.27% Auxiliant (plasticizer and lubricant) 2.63%3.33% Total: 100.00% 100.00%

Andalusite is used in the form of the commercially available productPurusite®

(Damrec). Graphite is used in the form of the commercially availableproduct Timrex® KS 44 (Timcal).

The resulting mixtures were extruded and further treated as describedabove in Example 1.

Example 6 Preparation of Ceramic Test Bodies of Different CompositionFor Determination of Porosity And Mechanical Strength

The raw materials listed in Table 8 were mixed in a conventional mixer(Eirich mixer) to obtain extrudable pastes. 12 hours after preparation,the pastes were extruded through a piston press to obtain rods of 8 mmdiameter.

TABLE 8 Composition for raw materials Raw materials FIDI 30 FIDI 30aFIDI30b With selected Andalusite >10 μm-55 μm Andalusite 53.8%  48.7% 47.7%  Graphite  13% 11.7%  11.5%  Binder 4.8% 4.5% 4.4% H₂O 0.0% 18.7% 31.8%  Titania dispersion 3.1% 0.0% 0.0% Alumina dispersion 10.2%  9.5%0.0% Anatase (pure TiO2) 0.0% 2.3% 0.0% Auxiliant (plasticizer and 5.1%4.9% 4.6% lubricant) Total: 100.00%   100.00%   100.00%  

Andalusite is used in the form of the commercially available productKerphalite® KF (Damrec). Graphite is used in the form of thecommercially available product Timrex® KS75 (Timcal). Titania is used inthe form of the commercially available product S5-300A (MilleniumChemicals, dispersion containing 20% by weight TiO₂). Alumina is used inthe form of the commercially available product Aerodisp® W 630 (Degussa,dispersion containing 30% by weight Al₂O₃). Anatase is used in the formof a powder having a BET surface of 11.7 m²/g; a mean particle diameterd₅₀ of 0.25 μm; and a d₉₀ of 0.77 μm. The percentages in the above tablerefer to the total mass amount of each compound in the extrudablemixture.

The green rods were stored for 2 days in an atmosphere of roomtemperature and a relative humidity of 80%. Subsequently, the rods weredried in a conventional drying oven.

The dried rods were subsequently subjected to the following heatingprogram:

-   -   heating from ambient temperature to 250° C. with a heating rate        of 0.5° C./min;    -   maintaining the temperature of 250° C. for two hours;    -   heating to 850° C. with a heating rate of 1.0° C./min;    -   maintaining the temperature of 850° C. for eight hours;    -   heating to the final sintering temperature of 1450° C. with a        heating rate of 2.0° C./min; and    -   maintaining the final sintering temperature of 1450° C. for two        hours.

The ceramic rods were then analyzed by qualitative X-ray diffraction(Rietveld analysis with a 30% ZnO standard); the results are summarizedin Table 10 below:

TABLE 9 Compositions of final ceramic bodies of Example 6 ComponentFIDI30 [%] FIDI30a [%] FIDI30b [%] 3:2 Mullite 72 69 65 Andalusite 13 1320 Amorphous 8 14 15 Tialite 7 4 0

The ceramic rods showed the porosity parameters as summarized below inTable 10 (determined by mercury diffusion as measured using a ThermoScientific Mercury Porosimeter—Pascal 140):

TABLE 10 Porosity parameters of final ceramic bodies of Example 7:Parameter FIDI30 FIDI30a FIDI30b Average Pore 15 13-14 18-19 Diameter[μm] Total Porosity [%] 58-59 53-54 57-59

In the standard three point CMOR test (cold modulus of rupture), theceramic bodies showed the mechanical strength (expressed by the breakingforce) given in Table 11:

TABLE 11 Mechanical strength of final ceramic bodies of Example 6:FIDI30 FIDI30a FIDI30b Breaking Force [N] 154 253 10

Table 10 shows that FIDI 30 and 30b have equivalent desirable porosity.However, the composition 30b without the titania/tialite, hasunacceptably low mechanical stability, as is shown in Table 11. FIDI30ahas even higher strength than FIDI30, which is caused by the fact thatthe titanium precursor is larger, namely a micro powder versus theFIDI30 which was manufactured with nano/colloidal solution instead.Additionally, FIDI30a shows a reduced porosity and pore size, which alsoincreases mechanical stability.

Example 7 Preparation of Ceramic Test Bodies of Different CompositionFor Determination of Porosity And Mechanical Strength

The raw materials listed in Table 12 were mixed in a conventional mixer(Eirich mixer) to obtain an extrudable paste. 12 hours afterpreparation, the pastes were extruded through a piston press to obtainrods of 8 mm diameter.

TABLE 12 Composition for raw materials (% by weight): Raw materials FIDI31 FIDI 31* FIDI 31a FIDI31b Andalusite 51.6 53.1 46.4 51.1 Graphite12.4 12.8 13.2 13.2 Binder 4.8 4.9 5.1 5.1 H₂O 5.4 2.8 18.2 1.0 Titaniadispersion 11.9 12.2 0.0 12.6 Alumina dispersion 10.1 10.3 10.6 10.6Anatase (pure TiO2) 0.0 0.0 2.5 2.5 Auxiliant (plasticizer 3.9% 3.9%3.9% 3.9% and lubricant) Total: 100.00 100.00 100.00 100.00

Andalusite is used in the form of a the commercially available productKerphalite® KF (Damrec), tailored to have a particle size range of fromabout 25 to 75 μm (d₁₀ to d₉₀). Graphite is used in the form of thecommercially available product Timrex® KS75 (Timcal). Titania is used inthe form of the commercially available product S5-300A (MilleniumChemicals, dispersion containing 20% by weight TiO₂). Alumina is used inthe form of the commercially available product Aerodisp® W 630 (Degussa,dispersion containing 30% by weight Al₂O₃). Anatase is used in the formof a powder having a BET surface of 11.7 m²/g; a mean particle diameterd₅₀ of 0.25 82 m; and a d₉₀ of 0.77 μm. The percentages in the abovetable refer to the total mass amount of each compound in the extrudablemixture.

The green rods were stored for 2 days in an atmosphere of roomtemperature and a relative humidity of 80%. Subsequently, they weredried in a conventional drying oven.

The dried rods were subsequently subjected to the following heatingprogram:

-   -   heating from ambient temperature to 250° C. with a heating rate        of 0.5° C./min;    -   maintaining the temperature of 250° C. for two hours;    -   heating to 850° C. with a heating rate of 1.0° C./min;    -   maintaining the temperature of 850° C. for eight hours;    -   heating to the final sintering temperature of 1450° C. (or 1500°        C.) with a heating rate of 2.0° C./min; and    -   maintaining the final sintering temperature of 1450° C. (or        1500° C.) for two hours.

The ceramic rods were then analyzed by qualitative X-ray diffraction(Rietveld analysis with a 30% ZnO standard); the results are summarizedin Table 13 below:

TABLE 13 Compositions of final ceramic bodies of Example 7 FIDI FIDIFIDI Component FIDI 31 [%] 31* [%] 31a [%] 31b [%] Firing Temp 1450 15001450 1500 1450 1500 1450 1500 (° C.) Extrusion 20-25 20-25 34-38 34-3810-13 10-13 40-44 40-44 pressure (bar) 3:2 Mullite 75 79 66 78 69 78 6769 Andalusite 11 — 10 — 10 — 10 — Amorphous 7 16 15 15 14 15 8 15Tialite 7 4 9 6 7 6 14 15 Rutile <1 1 <1 1 <1 1 1 1

The ceramic rods showed the porosity parameters as summarized below inTable 14 (determined by mercury diffusion, as measured using a ThermoScientific Mercury Porosimeter—Pascal 140):

TABLE 14 Porosity parameters of final ceramic bodies of Example 7:Parameter FIDI 31 FIDI 31* FIDI 31a FIDI 31b Firing Temp (° C.) 14501500 1450 1500 1450 1500 1450 1500 Average Pore 20.2 21.7 24.7 28.5 16.420.4 28.2 31.0 Diameter [μm] Total Porosity 57.9 53.8 57.5 58.2 62.558.7 57.2 56.1 [%]

The FIDI30 composition and a modified FIDI31b composition having thecomposition set forth in Table 15 below were also used to make ahoneycomb structure (firing temperature of 1500° C.) having a diameterof 14.5 cm and a length of 20 cm. The average pore diameter was 21.7 μmand the total porosity was 47.5%. Square shaped pillars having thelength of the honeycomb and a 6 cm×6 cm cross-section were cut from thehoneycomb, and then shortened to a length of 10 cm. A standard threepoint modulus of rupture (MOR) test was then performed along the axis ofthe samples. The force required to fracture the samples was 99N for thestructure made from the FIDI30 composition and 143N for the structuremade from the FIDI31b composition.

TABLE 15 Composition for raw materials (% by weight) for modifiedFIDI31b: Raw materials FIDI31b (modified) Andalusite 50.9 Graphite 13.1Binder 5.1 H₂O 1.0 Titania dispersion 12.5 Alumina dispersion 10.5Anatase (pure TiO2) 2.4 Auxiliant (plasticizer and lubricant) 4.6%Total: 100.0

The preceding description is merely exemplary of various embodiments ofthe present invention. Those skilled in the art will recognize thatvarious modifications may be made to the disclosed embodiments thatwould still be within the scope of the invention. The scope of theinvention is intended to be limited only by the appended claims.

1. A ceramic honeycomb structure comprising mullite and tialite, whereinthe volume ratio of mullite to tialite is 2:1 or higher.
 2. A ceramichoneycomb structure according to claim 1, wherein the volume ratio ofmullite to tialite is 4:1 or higher.
 3. A ceramic honeycomb structureaccording to claim 1, wherein the volume ratio of mullite to tialite is8:1 or higher.
 4. A ceramic honeycomb structure according to claim 1,wherein the volume ratio of mullite to tialite is 10:1 or higher.
 5. Aceramic honeycomb structure of claim 1, characterized in that themullite is a 3:2 mullite.
 6. A ceramic honeycomb structure of claim 1,characterized in that the tialite is enclosed by the mullite.
 7. Aceramic honeycomb structure according to claim 1, characterized in thatthe amount of mullite in the structure is greater than 50%, by volumecalculated on the basis of the total volume of the mineral phases.
 8. Aceramic honeycomb structure according to claim 1, further comprising oneor more solid mineral phases selected from the group consisting ofcordierite, andalusite, zirconia, titania, a silica phase, magnesiumoxide, magnesia alumina spinel, silicon carbide, and silicon nitride. 9.A ceramic honeycomb structure according to claim 8 comprising siliconcarbide, wherein the silicon carbide is present in an amount between 4and 30% by mass.
 10. A ceramic honeycomb structure according to claim 9,wherein the silicon carbide particle size is between 0.5 and 20 μm. 11.A ceramic honeycomb structure according to claim 8 comprising magnesiaalumina spinel, wherein the magnesia alumina spinel is present in anamount between 4 and 30%, by mass.
 12. A ceramic honeycomb structureaccording to claim 11, wherein the magnesia alumina spinel particle sizeis between 0.5 and 20 μm.
 13. A ceramic honeycomb structure according toclaim 1, comprising an andalusite phase of less than 10% by volume. 14.A ceramic honeycomb structure according to claim 1, characterized inthat the total pore volume of the structure is in the range between 30%and 70% calculated on the basis of the total volume of mineral phasesand pore space.
 15. A diesel particulate filter made using the ceramichoneycomb structure according to claim
 1. 16. A method for themanufacture of a ceramic honeycomb structure according to claim 1,comprising: providing a dried green honeycomb structure comprisingmullite and/or one or more mullite-forming compounds or compositions andtialite and/or one or more tialite-forming compounds or compositions;and sintering the dried green honeycomb structure.
 17. A methodaccording to claim 16, wherein the mullite-forming compound is selectedfrom the group consisting of kyanite, sillimanite, and andalusite.
 18. Amethod according to claim 17, wherein the mullite-forming compound isandalusite.
 19. A method according to claim 18, wherein the andalusitehas a particle size of from 0.1 μm to 100 μm.
 20. A method according toclaim 16, wherein the dried green honeycomb structure comprises tialite.21. The method according to claim 20, wherein the tialite is present inan amount between 2.5% to 15% by dry weight of the extrudable mixture.22. A method according to claim 16, wherein the tialite-formingcomposition is a mixture of titania and alumina and/or a mixture oftitania and one or more alumina precursors.
 23. A method according toclaim 22, wherein the alumina and/or the alumina precursor particle sizeis between 0.01 and 10 μm.
 24. A method according to claim 23, whereinthe alumina and/or alumina precursor is a colloidal or nanometricsolution.
 25. A method according to claim 22, wherein the titaniaparticle size is between 0.01 and 10 μm.
 26. A method according to claim22, wherein the amount of alumina and/or alumina precursors calculatedas Al₂O₃ is higher than the amount of titania.
 27. A method according toclaim 16, wherein the dried green honeycomb structure further comprisesgraphite.
 28. A method according to claim 27, wherein the medianparticle diameter (D50) of the graphite is between 1 and 100 μm.
 29. Amethod according to claim 16, wherein the dried green honeycombstructure further comprises silicon carbide or magnesium alumina spinel.30. The method according to claim 16, wherein the sintering step isperformed at a temperature between 1250° C. and 1700° C. 31-55.(canceled)